If `a-scene` has `vr-mode-ui` disabled the WebVR button is still shown

[gfodor]

It looks like aframe-xr always enables vr-mode-ui even if the scene has set it to off. The code here:

https://github.com/mozilla/aframe-xr/blob/50326e45f17fdfc87c1704cc7a16510058fba4e8/dist/aframe-xr.js#L192

Likely should inspect the value on the scene and use that value instead of the hardcoded true value for vr-mode-ui in the cases where it is being explicitly enabled. I'm happy to cut a PR if this makes sense!